Did Ebbetts tonight on an original Kickr in Erg mode. When done with each sprint it showed that on most of them I averaged 30w less than target. I tried different gearing and different cadences but did not make any difference. Is it asking to much for the trainer to react fast enough for 5 second bursts? Workout still kicked my butt.

ERG mode can definitely limit you during extremely short burst intervals. There is one thing you can try though that will make your ERG mode work better.

Go ahead and shift into your easiest possible gear next time. This means shifting to your small chainring in the front and the big cog on your cassette. By lowering the speed that your rear flywheel is spinning, it allows the ERG commands to be excecuted more quickly

But for 5 second bursts, you may never get a fast enough reaction. In these cases, you would be better off using resistance mode to successfully meet the target.